The benefit of adjuvant chemotherapy for resected pancreatic ductal adenocarcinoma (PDAC) has been confirmed in randomized controlled trials. For nonpancreatic periampullary cancers (NPPC) originating from the distal bile duct, duodenum, ampulla, or papilla of Vater, the role of adjuvant therapy remains largely unclear. This review describes methods for distinguishing PDAC from NPPC by means of readily available and recently developed molecular diagnostic methods. The difficulties of reliably determining the exact origin of these cancers pathologically also is discussed. The review also considers the possibility of unintentional inclusion of NPPC in the most important adjuvant trials on PDAC and the subsequent implications for interpretation of the results. The authors conclude that correct determination of the origin of periampullary cancers is essential for clinical management and should therefore be systematically incorporated into clinical practice and future studies.